Greetings
Hermit Crabs
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I have Blue-Leg, Scarlet and Hawaiian Zebra hermit crabs. They all seem to be equally good at clean-up crew duties, although the Scarlet's have grown noticeably faster than the others. The color of the Scarlet's, as well as the Hawaiian's is nice, but then again, so are the Blue-Leg's. It's just not as obvious unless you look closely...
They all seem to be equally agressive, at least in my tank, but not to an extreme. I have seen some gang warfare where they attacked a snail, but it was because the snail fell and didn't get turned right-side up quickly enough. Only once have I seen something that looked like they were killing a snail simply to take the shell. Survival of the fittest...
I typically buy a majority of Blue-Legs because you can get them for a good price (12 for $10 around here) and then add Scarlet and Zebra crabs for variety and color. I have to mail-order all except the Blue-Leg's and the shipping nearly doubles the cost.
Emerald Crabs
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When I established my tank, I bought a healthy sized clean-up crew that included 3 Emerald Crabs. I seldom saw the crabs and within 8 weeks, they were gone - never found a trace... Nothing else perished, so I was confused by the loss. During that time, I had a few spots of bubble algae, but the Emeralds never gave it a second look. About 4 months later, I bought a couple more Emerald Crabs to (hopefully) take care of a few small spots of bubble algae. Once again, they paid no attention to the algae and they were both missing in action within 4 weeks. I've given up on Emerald Crabs - they don't seem to be the bubble algae eaters that they reportedly are and they don't last in my tank
Take Care